In an image forming apparatus such as a copier or printer, generally a latent image is formed on a photosensitive drum, which is an electrostatic latent carrier, using a laser beam. Such an apparatus modulates the laser beam, which has been generated by a laser oscillator, in accordance with an image signal and irradiates the surface of a polygon mirror that is rotating at high speed. The laser beam is reflected by the rotating polygon mirror and is deflectively scanned along the axial direction (main-scan direction) of the photosensitive drum owing to such rotation of the mirror. The laser beam forms an image on the photosensitive drum via an imaging lens to thereby form an electrostatic latent image that conforms to each scanning line of the image.
A color image forming apparatus (tandem-type color image forming apparatus) known in the art has a plurality of image forming units each of which includes a laser scanning unit and a photosensitive drum. The apparatus forms images (toner images) of different colors by respective ones of the image forming units and transfers these images to a print medium in superimposed form. In a color image forming apparatus of this kind, it is necessary that the images formed by the image forming units be transferred to the print medium in a form in which the images are accurately superimposed on one another. That is, it is necessary to accurately adjust the starting point of image formation in the main-scan direction (the direction in which the laser beam is scanned across the photosensitive drum) and sub-scan direction (a direction approximately perpendicular to the main-scan direction and corresponding to the transport direction of the print medium) of each image on each photosensitive drum.
There are instances where the toner images are not in proper registration when they are superimposed on the transfer belt or recording medium. Such color misregistration is caused by a deviation in the irradiation position of each color, which is ascribable to mechanical installation error between each photosensitive body (photosensitive drum) and a scanning-type optical device (laser saner unit) and the mechanical mounting error of each of the optical components, and by driving non-uniformity of the photosensitive bodies and intermediate transfer belt (recording medium).
In an arrangement having a plurality of scanning-type optical devices provided in conformity with a plurality of photosensitive bodies, it has been proposed to reduce color misregistration in the sub-scan direction by accurately adjusting the rotational phase of each polygonal mirror in accordance with phase error exhibited by a beam detection signal of each scanning-type optical device (see the specification of Japanese Patent Application Laid-Open No. 2000-141750).
A conventional image forming apparatus provided with scanning-type optical devices the number of which is the same as the number of photosensitive bodies is inevitably high in price and large in size. In order to lower cost and reduce size, a scanning-type optical device that makes common use of a single polygonal mirror while using a plurality of light sources is advantageous. In the scanning-type optical device, light beams from a plurality of laser sources are concurrently deflected and scanned by the single polygonal mirror and expose on corresponding photosensitive bodies.
With such a scanning-type optical device, however, the technique set forth in the above-mentioned specification cannot be applied in order to adjust color misregistration. The reason for this is that since common use is made of a single polygonal mirror, it is theoretically impossible to adjust the rotational phase of the polygonal mirror on a color-by-color basis. Accordingly, adjustment of color misregistration must be performed by changing the mirror surface of the polygonal mirror for every light beam of each color. With this method, however, a problem which arises is that a maximum of one pixel of color misregistration (one scanning-line interval) is produced along the sub-scan direction.
A feature of the present invention is to solve this problem and at least one other problem of the prior art. Other problems of the prior art will be understood through a reading of the entire specification.